1. Field of the Invention
The present invention relates to drive control technology for a brushless motor.
2. Description of the Related Art
One brushless motor known in the art to date is that disclosed in JPA 2001-298982, for example.
In conventional brushless motors, it is common practice to employ a rectangular waveform for the drive signal. Where a drive signal of rectangular waveform is used, there is a risk that the motor will overheat due to overcurrent generated when the drive signal reverses polarity. Thus, conventional brushless motors are typically provided with a circuit for limiting overcurrent.
FIG. 23 illustrates one example of an overcurrent limiting circuit used in a conventional brushless motor. In this circuit, an overcurrent detection transistor PT and an overcurrent sensor ECS are provided on the ground side of a bridge circuit HB which drives the coil C of the brushless motor. When overcurrent flows to the bridge circuit HB, the overcurrent sensor ECS will detect the flow of overcurrent. Then, in response of an output signal from the overcurrent sensor ECS, the drive circuit will limit the application voltage or application current to the coil C.
Preferably, such limiting of overcurrent will properly take place only during abnormal circumstances, such as where a load has become excessively large. However, since current flow is rather high during startup of the motor, the current limiting function may kick in during startup, resulting in undue limiting of current. Where current is limited during startup in this way, a resultant problem is that adequate torque will not be produced in some instances.